V. Chandrasekhar

Mesoscopic ferromagnet-superconductor junctions and the proximity effect

We have measured the electrical transport of submicron ferromagnets (Ni) in contact with a mesoscopic superconductor (Al) for a range of interface resistances. In the geometry measured, the interface and the ferromagnet are measured separately. The ferromagnet itself shows no appreciable superconducting proximity effect, but the ferromagnet/superconductor interface exhibits strong temperature, field, and current bias dependences. These effects are dependent on the local magnetic field distribution near the interface arising from the ferromagnet. We find that the temperature dependences can be qualitatively described by a modified version of the Blonder-Tinkham-Klapwijk theory for normal-superconductor transport.

Application of commercially available cantilevers in tuning fork Scanning Probe Microscopy (SPM) studies

We describe the construction and operation of an atomic force microscope based on a tuning fork sensor with probe tips obtained from commercially available cantilevers. Only the tip from the cantilever is mounted on the tuning fork, minimizing the change in resonant frequency and the quality factor Q of the tuning fork. The technique allows the use of a variety of commercially available tips for use in different probe microscopy applications like magnetic force microscopy. The compact design of the microscope and its simple feedback electronics make it well suited for operation in novel environments such as at cryogenic temperatures. We present high resolution atomic force images of graphite surfaces, as well as scanning probe images of magnetic domains and Ni dot arrays taken with this microscope.

Kondo effect and dephasing in low-dimensional metallic systems

Preface V. Chandrasekhar, et al. Introduction N.O. Birge, et al. Finite Size Effects in Kondo Alloys. Effect of Disorder on the Kondo Behavior of Thin Cu(Mn) Films T.M. Jacobs, N. Giordano. The Kondo Effect and Weak Localization P. Phillips, I. Martin. Surface Magnetic Anisotropy of Kondo Impurities Induced by Spin-Orbit Scattering O. Ujsaghy. Finite Size Effects in Spin Glass Alloys. Thermopower of Mesoscopic Spin Glasses C. Strunk, et al. Shape-Induced Magnetic Anisotropy in Dilute Magnetic Alloys V.N. Gladilin, et al. Zero-Bias Transport Anomaly in Metallic Nanobridges H.B. Weber, et al. Conductance Noise and Irreversibility in Diluted Magnetic Semiconductors J. Jaroszynski, et al. Point Contact Spectroscopy and Tunnelling Spectroscopy of Kondo Impurities. Enhancement of Kondo Temperature in Nanometer-Size Point Contacts I.K. Yanson, et al. Scanning Tunnelling Spectroscopy of a Single Kondo Impurity R. Berndt, W.-D. Schneider. Two-Level Systems and Dephasing in Thin Metal Structures. Two-Channel Kondo Effect from Tunneling Impurities G. Zarand. Electron Decoherence at Zero Temperature P. Mohanty. Probing Interactions in Mesoscopic Gold Wires F. Pierre, et al. Kondo Effect in Non-Equilibrium J. Kroha. Kondo Effect in Quantum Dots. Tunneling Through a Quantum Dot A. Schiller. Electron Transport Through Quantum Dots: An Unusual Kondo Effect S. De Franceschi, et al. The Kondo Effect in a Single-Electron Transistor D. Goldhaber-Gordon, et al. Contributions Related to Poster Presentations. Flux Dependent Dielectric Response of Stacked Nanoscopic Rings K.-H. Ahn, P. Fulde.High-Frequency Response of Two-Level Systems in NixNb1-x Metallic Point Contacts O.P. Balkashin, et al. Giant Magnetoresistance of a Single Interface J. Balogh, et al. Fermi Edge Singularities in Transport Through Quantum Dots E. Bascones, et al. Theory of Magnetoresistance in Films of Dilute Magnetic Alloys L. Borda. Current and Shot Noise in a Ferromagnetic Double Tunnel Junction with an Atomic Size Spacer B.R. Bulka, et al. Positive Domain-Wall Magnetoresistance of Ferromagnetic Point Contacts J. Caro, et al. Enhancement of Kondo Effect Due to Spin-Singlet-Triplet Competition in Quantum Dots M. Eto, Y.V. Nazarov. Diagrammatic Theory of the Anderson Impurity Mod with Finite Coulomb Interaction K. Haule, et al. Generalized Conductance Sum Rule in Atomic Break Junctions S. Kirchner, et al. Is CeNiSn a Kondo Semiconductor? Y.G. Naidyuk, et al. The Influence of Single Magnetic Impurities on the Conductance of Quantum Microconstrictions A. Namiranian, et al. 7 Additional Chapters.

Proximity effect thermometer for local electron temperature measurements on mesoscopic samples

Using the strong temperature-dependent resistance of a normal metal wire in proximity to a superconductor, we have been able to measure the local temperature of electrons heated by flowing a direct-current (dc) in a metallic wire to within a few tens of millikelvin at low temperatures. By placing two such thermometers at different parts of a sample, we have been able to measure the temperature difference induced by a dc flowing in the samples. This technique may provide a flexible means of making quantitative thermal and thermoelectric measurements on mesoscopic metallic samples.

Reentrance effect in normal-metal/superconducting hybrid loops

We have measured the transport properties of two mesoscopic hybrid loops composed of a normal-metal arm and a superconducting arm. The samples differed in the transmittance of the normal/superconducting interfaces. While the low transmittance sample showed monotonic behavior in the low temperature resistance, magnetoresistance and differential resistance, the high transmittance sample showed reentrant behavior in all three measurements. This reentrant behavior is due to coherent Andreev reflection at the normal/superconducting interfaces. We compare the reentrance effect for the three different measurements and discuss the results based on the theory of quasiclassical Greens functions.

Influence of temperature-dependent inelastic scattering on the superconducting proximity effect

We have measured the differential resistance of mesoscopic gold wires of different lengths connected to an aluminum superconductor as a function of temperature and voltage. Our experimental results differ substantially from theoretical predictions which assume an infinite temperature-independent gap in the superconductor. In addition to taking into account the temperature dependence of the gap, we must also introduce a temperature-dependent inelastic scattering length in order to fit our data.

Extrinsic origins of electrical transport anomalies near the superconducting transition in mesoscopic aluminum lines

We investigate whether inadvertent noise currents may induce the previously reported resistive peak and negative differential resistance (dV/dI) anomalies near the superconducting transition in quasi-one-dimensional aluminum lines. Although our samples show no intrinsic anomalies, low frequency current deliberately applied to the sample in addition to the ordinary measuring current induces the anomalies. In our work the resistive peak results from simple mixing, but the negative dV/dI feature arises from a more complex effect.

Resistance anomalies in mesoscopic superconducting Al structures

We show that the recently reported anomalous resistance peaks near the superconducting transition of quasi-one-dimensional mesoscopic Al structures can be induced by applying either radio-frequency radiation or a sufficiently high dc current. The nonmonotonic R(T) curves can be linked to changes in the voltage-current characteristics below the superconducting transition. The observed dependence on the position of the voltage probes is consistent with recent theoretical calculations which interpret the anomalies in terms of the charge imbalance near phase-slip centers created at specific spots on the sample.

Superconducting proximity effect in the presence of strong spin scattering

We report measurements of the four terminal temperature dependent resistance of narrow Au wires implanted with 100 ppm Fe impurities in proximity to superconducting Al films. The wires show an initial decrease in resistance as the temperature is lowered through the superconducting transition of the Al films, but then show an increase in resistance as the temperature is lowered further. In contrast to the case of pure Au wires in contact with a superconducting film, the resistance at the lowest temperatures rises above the normal state resistance. Analysis of the data shows that, in addition to contributions from magnetic scattering and electron-electron interactions, the temperature dependent resistivity shows a substantial contribution from the superconducting proximity effect, which exists even in the presence of strong spin scattering.

New phenomena in nanostructured materials

In recent years, a level of nanoscale fabrication has been achieved where dimensions and energy scales are such that quantum effects are dominant and can even be controlled in a variety of condensed matter systems. In this paper, several examples are given to illustrate the influence of nanostructuring on the physical properties of superconducting and normal magnetic systems. For superconductors we will discuss quantum interference effects in mesoscopic single loops and multiloop structures and the confinement of flux lines by a lattice of artificial submicron antidots. For magnetic layers we will analyze the size dependence of the Kondo and spin-glass effects.